Drive the value of a Follow Path constraint
Right-click on the Offset value of your constraint.  Add driver ->
Scripted Expression (don't worry, no script needed) 
Open a Driver Window (Shift F6)

In the driver select the armature as the object, and the control
bone as the bone.
Choose Y location and local space.
The expression should just be var, nothing behind it.
Move the controller in local y direction (in your scene global z)